1. Field of the Invention
This invention relates to electromagnetic beam transmission/reception systems, and more particularly to radar systems employing active transmit/receive (T/R) modules for directional transmission and reception applications such as monopulse tracking.
2. Description of the Prior Art
An ability to detect slight angular variations from a reference direction is required for an effective radar tracking system. A typical tracking radar has a narrow beam in at least one dimension to receive echoes from a target, and thereby track the target in that dimension. An early technique for radar tracking involved sensing the target location with respect to the radar antenna axis by rapidly switching the beam from one side of the axis to the other. By observing an oscilloscope that displayed the video return from the two beam positions side-by-side, the target's angular position relative to the axis could be determined. With the target on-axis, the two pulses were of equal magnitude; when the target moved off axis, the two pulses became unequal. When a pulse inequality was observed, the operator could reposition the antenna to regain a balance and thus track the target. This technique is called sequential lobing.
The above technique was later extended to the continuous rotation of a pencil beam about the target. Error signals proportional to the angular tracking error, with a phase or polarity indicating the direction of the error, were generated and used to actuate a servosystem that drove the antenna in the proper direction to reduce the error to zero. This technique is called conical scan.
The susceptibility of these techniques to echo amplitude fluctuations led to the development of a tracking radar that provided all of the necessary lobes for angle-error sensing simultaneously. By comparing the output from the lobes simultaneously on a single pulse, the effect of echo amplitude fluctuations over time was eliminated. This technique became known as monopulse, referring to its ability to obtain complete angle error information with a single pulse. While developed in connection with tracking radar, the monopulse approach is also used in other systems such as homing devices, direction finders and some search radars. The sequential lobing, conical scan and monopulse techniques are well known, and are described for example in Radar Handbook (2d Ed.) by Merrill Skolnik, Chapter 18 by Dean Howard "Tracking Radar", pp. 18.1-18.22, McGraw-Hill Publishing Co., 1990.
FIG. 1 is a block diagram of a conventional azimuth monopulse tracking radar system. A pair of "feed horns" or radar transmission/reception elements 2 are shown, although in a practical system a more sophisticated antenna would be employed. With the system in a RECEIVE mode, the radar signals received by the two elements are fed to a sum-and-difference processor 4 that compares the outputs from the two elements to sense any imbalance in the azimuth direction of the received radar signals with respect to their center axis. Sum-and-difference processor 4 is typically implemented with a hybrid T or magic T waveguide device that produces a sum (.SIGMA.) output representing the in-phase portion of the two received signals, and a difference (.DELTA.) output representing any phase difference between the two received signals. If the received echo signals are identical, the .SIGMA. output is unity and the .DELTA. output is zero; the .DELTA. output increases rapidly, and the .SIGMA. output decreases slowly, as the two received signals differ more and more. The signals differ when the target is not exactly in line with a perpendicular to the two feed horns 2, i.e., when the target is off-boresight with respect to the feed horns in azimuth.
The .SIGMA. and .DELTA. signals are converted to intermediate frequency (IF) by mixers 6, 8, using a common oscillator 10 to maintain relative phase at IF. The IF .SIGMA. signal is amplified in IF amplifier 12 and detected by amplitude detector 14 to provide a video input to range tracker 16 via a video amplifier 18. The range tracker 16 determines the time of arrival of the desired target echo, and provides gate pulses which turn on portions of the radar receiver only during the brief period when the desired target echo is expected.
The .DELTA. signal is amplified by IF amplifier 20, whose output is applied along with the output of .SIGMA. IF amplifier 12 to a phase detector 22. The phase detector produces an output azimuth angle error signal in the form .DELTA.sin.THETA./.SIGMA., where .THETA. is the phase angle between the .SIGMA. and .DELTA. signals. With the radar properly adjusted, .THETA. is normally either 0.degree. or 180.degree.. The function of the phase detector is to provide a + or - polarity, giving a directional sense to the azimuth error output.
In a pulsed tracking radar the azimuth error output is bipolar video, that is, a video pulse with an amplitude proportional to the angular error and a polarity that corresponds to the direction of the error. This video signal is typically processed by a boxcar circuit (not shown) which charges a capacitor to the peak video pulse voltage and holds the charge until the next pulse, at which time the capacitor is discharged and recharged to the new pulse level. With moderate low-pass filtering, a DC error voltage output is produced that is used by the system's servo-amplifiers to correct the antenna positions and track the target.
The gated video signal from range tracker 16 is used to generate a DC voltage proportional to the magnitude of the .SIGMA. signal for an automatic gain control (AGC) circuit 24 in both IF amplifier channels. The AGC maintains a constant angle tracking sensitivity, even though the target echo signal varies over a very large dynamic range, by controlling gain or dividing by .SIGMA.. AGC is necessary to keep the gain of the angle tracking loops constant for stable automatic angle tracking.
During the TRANSMIT mode, an exciter 26 generates the waveform to be radiated. This signal is processed through power amplifier 28, and routed through a duplexer 30 to the radiating elements 2 via the sum channel. The duplexer 30 acts as a passive directional rapid switch to protect the receiver from damage when the high power transmitter is on; during RECEIVE it directs the weak received signal through the receiver section rather than to the transmitter.
While only two radiating/receiving elements 2 are shown in FIG. 1 for simplicity, complex systems can have a much larger number of elements. A more complex system is illustrated in FIG. 2, in which a much larger number of radiating/reception elements are shown divided into numerous sets 32 of plural elements each; the total number of elements can be in the hundreds or more. Although illustrated as a linear array for simplicity, the elements could be arranged in a cylindrical array or any other desired geometric format, e.g., an arc. The sets of elements are organized into sectors, with each set within a given sector transmitting or receiving at a given time. For purposes of illustration, 16 sets 32 of elements are shown, arranged in 13 sectors of 4 sets each. If the array shown were cylindrical, 16 sectors would be selectable. Three successive sectors are identified by reference numerals 34, 36 and 38.
This type of array is scanned by switching in and switching out connections between the transmitter/receiver and different sectors. The selection of sectors can jump from one part of the array to another, or can progress from each sector to the next adjacent one, e.g., 34, 36 to 38. In the latter case, a set of elements at one side of the sector is switched off and the next set of elements immediately on the other side of the sector is switched on each time the selected sector is advanced.
A centralized transmitter 40 and receiver 42 are connected via a duplexer 43 and feed network 44 to a set of switches 46a, 46b, 46c, and 46d. Each switch is an M pole device, where M is the number of element sets 32, and can be connected to any of the different sets. To make a connection between a particular desired sector and the signal feed network 44, each switch is set to a different one of the 4 sets of elements 32 within the desired sector. For example, switch connections to sectors 34, 36 and 38 are indicated by solid, dashed and dotted lines, respectively. If more sets of radiating elements 32 are employed, there is a corresponding increase in the number of terminals within each switch. The total number of switches will equal the number of element sets within each sector. To scan from one sector to the next, the setting of each switch is advanced by one terminal per sector. With the 16 sets of elements illustrated, a full 360.degree. scan around a cylindrical array requires that the switches be advanced through 16 successive sectors, each 221/2 apart.
The switches are generally electromechanical or PIN diodes, which are both subject to failure. The failure rate per radar system increases, and the system reliability drops correspondingly, as the number of switches is increased.
In contrast to the centralized feed system of FIG. 2, active transmit/receive (T/R) modules have been developed recently that can generate RF power directly at the antenna elements, set relative phase relationships between the elements, and amplify a received signal. Locating the modules at each antenna element in the antenna array simplifies the problem of scanning non-linear/non-planar array configurations without a central RF power source.
Active T/R modules are described, for example, in Fisher, "GaAsIC Application's in Electronic Warfare, Radar and Communication Systems", Microwave Journal, May, 1988, pp. 275-292. They have low RF losses, a low vulnerability to interference, and distributed rather than centralized power generation. However, simply adding T/R modules to the system of FIG. 2 would involve providing a module at each of the antenna elements while still having to connect all of the modules to a central signal processing location, and switching between the sets of modules. Even with the illustrative total of 16 sets and 4 sets per sector, this would still require the use of 4 different 16-way switches with their attendant reliability problems.